LONDON, Sept 14 (Reuters) - The new leader of Britain's opposition Labour Party named an ally whose declared aim is the overthrow of capitalism as finance spokesman on Monday, pushing hard-left policies that opponents say will make the party unelectable.

After winning Labour's top job by a landslide on Saturday, Jeremy Corbyn named senior members of his would-be government including John McDonnell - a former trade unionist who backs renationalising banks and imposing wealth taxes - as his finance minister-in-waiting.